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[Bug dynamic-link/21258] New: Branch predication in _dl_runtime_resolve_avx512_opt leads to lower CPU frequency


https://sourceware.org/bugzilla/show_bug.cgi?id=21258

            Bug ID: 21258
           Summary: Branch predication in _dl_runtime_resolve_avx512_opt
                    leads to lower CPU frequency
           Product: glibc
           Version: 2.26
            Status: NEW
          Severity: normal
          Priority: P2
         Component: dynamic-link
          Assignee: unassigned at sourceware dot org
          Reporter: hjl.tools at gmail dot com
  Target Milestone: ---
            Target: x86-64

On Skylake server, _dl_runtime_resolve_avx512_opt is used to preserve
the first 8 vector registers.  The code layout is

  if only %xmm0 - %xmm7 registers are used
     preserve %xmm0 - %xmm7 registers
  if only %ymm0 - %ymm7 registers are used
     preserve %ymm0 - %ymm7 registers
  preserve %zmm0 - %zmm7 registers

Branch predication always executes the fallthrough code path to preserve
%zmm0 - %zmm7 registers speculatively, even though only %xmm0 - %xmm7
registers are used.  This leads to lower CPU frequency.

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